In the last few decades, the use of natural fibres to reinforce polymer composites has been increasing because of their sustainability, renewability, biodegradability, low thermal expansion, manufacturer-friendly attributes such as low density and abrasiveness, excellent mechanical properties such as very high specific stiffness and strength and consumer-friendly attributes such as lower price and higher performance. A typical natural microfibre consists of bundles of nanofibres which in turn consist of several or more elementary (primary) nanofibrils formed by cellulose chains (a homopolymer of glucose), concreted by/in a matrix containing lignin, hemicellulose, pectin and other components. The diameter of primary cellulose nanofibrils is typically in the range 3-4 nm. The nanofibrils consist of monocrystalline cellulose domains linked by amorphous domains. Amorphous regions act as structural defects and can be removed under acid hydrolysis, leaving cellulose rod-like nanocrystals, which are also called whiskers, and have a morphology and crystallinity similar to the original cellulose fibres. Depending on the source of cellulose, the cellulose content varies from 35 to 100%. These fibres, isolated in their primary nanofibrillar form exhibit extraordinarily higher mechanical properties (stiffness/strength) than at the microscale (as bundles of nanofibres) or in their natural state. In recent years, these nanocrystalline cellulose fibres have been explored as biologically renewable nanomaterials that can be applied in several engineering applications. While numerous methods have been explored for the production of microfibrillated cellulose (MFC), which by definition (Reference: Robert J. Moon, Ashlie Martini, John Nairn, John Simonsen and Jeff Youngblood, ‘Cellulose nanomaterials review: structure, properties and nanocomposites’ Chem. Soc. Rev., 2011, 40, 3941-3994), consists of cellulose fibres with diameters in the range of 20-100 nm and a length in the range between 0.5 μm and tens of microns, the production of nanofibrillated cellulose (NFC), and cellulose nanocrystals (CNCs) is more challenging due to the requirement to separate or deconstruct the cellulose fibres and/or crystals to a much greater degree. Attempts to date to produce these two types of nanocellulose (CNCs and NFCs) have focussed on the use of chemical, physical, mechanical and enzymatic steps as pre-treatments between conventional pulping processes and final mechanical defibrillation processing alone or in combinations thereof. For NFC, the prior art refers to a fibre diameter in the range of 3-20 nm and a length in the range between 0.5 and 2 μm. These nanofibrils can be further made up of primary cellulose nanofibrils typically having a diameter of 3-4 nm. For example, a cellulose nanofibril with a diameter of 10 nm may consist of a bundle of a few primary cellulose nanofibrils with 3-4 nm diameter. For CNC, the prior art refers to fibre/crystal diameters/widths in the range of 3-20 nm and lengths of up to 500 nm (except the special example of tunicate CNCs or t-CNCs, which have a higher aspect ratio).
A typical procedure for isolating nanocrystals of cellulose relies on acid hydrolysis using corrosive acids (like H2SO4 and HCl), followed by centrifuging, dialysing, ultrasonication and drying (a typical flowchart showing this process is shown in FIG. 1). Depending on the cellulose source and hydrolytic conditions, cellulose nanocrystals (CNCs) with the diameter range of 3-15 nm and length in the range of 50-500 nm are isolated. Some of these products are produced at semi-commercial scale (e.g. 1 tonne per day) using wood fibres as the raw material. High aspect ratio cellulose nanocrystals (with an aspect ratio of 65-100) can be obtained from rare marine animals called tunicates (urochordates), but this is not a commercially viable or sustainable route. Therefore, the sustainable production of nanocrystals that are of a higher aspect ratio, or closer to that of CNCs derived from tunicates (t-CNCs), and doing so from plant source materials, remains a challenge.
For isolation of microfibres, which are called microfibrillated cellulose (MFC) with diameters in the range of 20-100 nm and length in the range of 0.5-10's μm, mechanical methods such as ultrasonication, homogenisation, milling, grinding, cryocrushing, or combinations of these are widely used to defibrillate the macroscale bleached pulp fibres into MFC fibrils which essentially consist of bundles of nanofibrils. In order to further refine and separate the MFC into its constituent nanofibrils and to isolate these further thinner particles called nanofibrillated cellulose (NFC) or cellulose nanofibrils (CNF), with diameters in the range of 3-20 nm and lengths in the range of 500-2000 nm, a significantly larger amount of mechanical energy typically needs to be applied than that required to refine material to the microfibrillar level. In reported methods, additional chemical or enzymatic pre-treatments applied after pulping and bleaching but prior to mechanical processing are usually claimed to be beneficial for reducing both mechanical energy consumption and resultant nanofibre diameter, as the chemical agents can aid in the removal of matrix materials such as lignin and hemicellulose that bind the fibres together. FIGS. 2A and 2B compare two typical procedures used in the art for producing MFC and NFC, respectively.
Delignification and bleaching are chemical processes widely used in the paper manufacturing industry and are key steps in the pulping process.
When a large amount of mechanical energy is applied to a cellulosic feedstock or the cellulose is exposed to harsh chemical pre-treatments, the cellulose fibres can be prone to breakage, thereby reducing their length and aspect ratio. Therefore, the production of nanocellulose is typically governed by a delicate balance between the requirement to input sufficiently large amounts of energy in order to isolate the nanofibres and the propensity of this large amount of energy to break fibres, thereby reducing their length and aspect ratios. Consequently, efforts to manufacture nanocellulose at commercial scale have been hindered by the high cost introduced by these additional processing steps and the challenge of avoiding fibre breakdown during processing. In manufacturing nanocellulose, mechanical processing is typically performed by passing a cellulosic feedstock through a mechanical processing step a number of times to facilitate the gradual breakdown of the cellulose to its nanoscale fibrils. For example, cellulosic feedstock material may be passed through equipment such as a homogeniser or disc refiner several times or more before the cellulose is sufficiently separated that predominantly nanofibres are yielded. In a commercial process, this requirement to pass the material through the same step multiple times can result in high energy costs and long processing times, reducing the commercial attractiveness of the process. Some examples of typical processing conditions disclosed in the patent literature for producing cellulose nanofibrils, including the number of passes through a particular mechanical processing step are set out in Table 1 below:
TABLE 1Different mechanical methods for theproduction of cellulose nanofibrilsReference/PatentMethodsCommentsU.S. Pat. No.Temperature assistedObtained type:4,374,702homogenisation (8-20MFC (no fibrepasses)diameter isreported)U.S. Pat. No.Rubbing (shear)MFC (no fibre6,183,596 &Supergrindingdiameter isU.S. Pat. No.High pressurereported)6,214,163homogenisationU.S. Pat. No.Double disc refinerMFC was produced7,381,294 &(shearing) (up to 80with diameter >0.1WO 2004/009902passes)μm after 15 passesU.S. Pat. No.Alkaline pre-treatmentNFCs are produced,5,964,983and acid hydrolysisafter grinding orcoupled with screeninghomogenising (8-10or homogenisationpasses) after acidhydrolysis at60-100° C.WO 2007091942Enzymatic pre-treatmentNFCs are producedand high pressureafter 5 passeshomogenisationUS 2008/0057307 &Low shear refiningNFCs are producedU.S. Pat. No.followed by high shearafter 7 passes7,566,014refining orhomogenisationWO2012/097446 &Double disc refinerNFCs are producedUS 2011/0277947after 8 passes
Some processes for the manufacture of cellulose nanofibrils use a chemical pre-treatment called TEMPO oxidation in which a cellulose pulp is exposed to TEMPO [(2,2,6,6-Tetramethylpiperidin-1-oxyl (CAS No: 2564-83-2). This pre-treatment loosens the nanofibrils, making it easier to defibrillate them from each other in subsequent mechanical processing. TEMPO processing enables 3-4 nm diameter nanofibrils to be obtained, however TEMPO agents are expensive and toxic, making their use and disposal difficult. In addition, the use of TEMPO agents results in conversion of the surface of the nanofibrils from one dominated by hydroxyl groups to one dominated by carboxyl groups. This can be a disadvantage when modification of the cellulose surface chemistry for some applications requires a hydroxylated surface.
Literature published before 2011 tends to use the terms MFC and NFC interchangeably, with these terms being used for both nanofibrils and microfibrils. In this specification, we distinguish between MFCs and NFCs, using the definitions given by Moon et. al. Chem Soc. Review’ 2011. Throughout this specification, the terms “MFC” (microfibrillated cellulose) and “CMF” (cellulose microfibre) are used to describe fibrils, including bundles of nanofibrils, with a diameter above 20 nm and length in 10 s of microns. The terms “NFC” (nanofibrillated cellulose) and “CNF” (cellulose nanofibre) are used to describe nanofibrils having a diameter between 3 to 20 nm. The NFCs obtained by the present invention are significantly longer than NFCs described in the prior art and may have a length above 500 nm up to 7 microns or longer. The term “CNC” is used to describe cellulose nanocrystals, which are rod-like or whisker shaped particles that are typically produced after acid hydrolysis of bleached pulp, MFC or NFC. CNCs with a high aspect ratio (3-5 nm diameter, 50-500 nm in length), are essentially 100% cellulose and are highly crystalline (54-88%). The CNCs obtained via acid hydrolysis in the present invention are longer (up to 1.5-2 microns or longer) than CNCs obtained in the prior art.
Commercial nanocellulose production largely uses wood as a source of cellulose due to wood's abundance, availability in commercial quantities and given that much of the development of nanocellulose has been supported by the forestry industry, motivated by a desire to find new applications for wood.
In one aspect, the present invention relates to producing NFC with lowest possible energy (that is generally used for MFC production). As stated in paragraph [0002] of this specification, production of NFC and CNC (cellulose nanocrystals) is more difficult than production of MFC due to the requirement to separate or deconstruct the cellulose fibres to a much greater degree. This typically results in the cellulose fibres being broken, resulting in the length of the fibres becoming significantly shorter and thus reducing the aspect ratio of the fibres.
In general, the prior art discloses that manufacturing processes requiring high energy input, disadvantaged by clogging problems during mechanical processing, complex recovery methods, harsh chemical treatments and/or high energy mechanical treatments are required to produce nanocellulose materials.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.